1. Field of the Invention
The present invention relates to a radio communication system in which data is transmitted on a radio transmission line.
2. Description of the Prior Art
In a radio communication system in which data is transmitted on a radio transmission line from a transmitter unit and the transmission data is demodulated and reproduced in a receiver unit, a carrier signal is digitally modulated according to data to be transmitted. A variety of methods have been proposed and put into practical use for such digital modulation. The present invention relates primarily to a quadrature amplitude modulation (QAM) method.
In a QAM radio commmunication system, the carrier is modulated in phase and, at the same time, in amplitude according to data to be transmitted. The data is thereby transmitted as one of many corresponding coded points arranged on an imaginary plane. This enables a large amount of data to be transmitted at one time and thus significantly increases the transmission capacity of a radio transmission line.
According to the QAM method, many coded points corresponding to data are created, as mentioned above. The number of coded points may be 16, 32, 64, and sometimes as much as 128 and so on. In the QAM method, one of the significant functions of the receiver unit is the recovery of a reference carrier from the transmission signal, which is used for synchronous detection. That is, the thus recovered carrier is used to demodulate the transmission data and reproduce the original data.
The most frequently used QAM method is the 16-level method. In this case, a process identical to a known process under a typical 4-phase phase shift keying (PSK) method is usually utilized for recovering the reference carrier. That is, the so-called multiply method, such as the two-multiply or four-multiply method, may be used for realizing the process. In this case, a phase selection control process is performed in a carrier recovery circuit of the receiver unit.
In the higher level QAM methods, such as the 16-level method, a problem arises when using the prior art carrier recovery circuit in suppressing so-called jitter (fluctuation) of the reference carrier. With 32-, 64- and 128-level QAM methods, recovery of a carrier for synchronous detection, using the prior art carrier recovery circuit, is impossible for all intents and purposes.
In view of the above, it is desired to realize a QAM radio communication system for a multilevel QAM method of 32-level or higher.